This idea has definitely crossed our minds, but it poses two problems:

• There's no way to implement it without a server handler that reconstructs the file from the slices. I'm not aware of any web server that does this on its own.
• Older browsers lack built-in support for the File API. Working around this requires using Flash/Silverlight.

These put it squarely outside of the Kendo UI scope, as it is a client-side framework based on standard HTML features.